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Author Topic: CW Filters  (Read 367 times)
VK3GDM
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Posts: 28




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« on: August 01, 2007, 06:10:22 PM »

Hi All,

K7PEH's topic on CW filters for the IC-706 have trigger my question:

I have a 400Hz CW IF filter (@9MHz) from INRAD for my IC-728.  It seems to work as expected, but the signal strength as indicated by the s-meter drops quite significantly (About 10dB) when switching from the normal SSB IF filter to the narrow filter.  Is this normal behaviour?

Also, the CW tone produced in narrow mode is a little distorted,  which is to be expected, I believe.

73
David Mayes
VK3GDM
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N8UZE
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Posts: 1524




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« Reply #1 on: August 01, 2007, 06:18:56 PM »

You might not have the signal quite tuned in.  Many times the pitch that you like and tend to tune to isn't the actual pitch of the side tone generated by the radio.  When you kick in the filter try tuning back and forth until the signal strength peaks.  With a 400Hz filter you should have little to no distortion if you are tuned in correctly.  The distortion tends to occur with the much narrower filters.
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N4KZ
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Posts: 590




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« Reply #2 on: August 02, 2007, 07:08:38 AM »

Switching from a SSB filter down to a very narrow CW filter will often produce a lower S-meter reading because you have reduced the amount of atmospheric noise being received. The degree of S-meter drop will be effected by the strength of the incoming signal with strong signals dropping less on the meter than weak signals.

As for distortion, slowly tune back and forth across the signal. What sounds to be distortion might actually be caused by you not being tuned to the correct frequency -- the audio frequency -- not the frequency on the readout. Always tune by ear -- not what the frequency readout says. (Of course, you must stay inside the band!)

73, N4KZ
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WB2WIK
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Posts: 20537




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« Reply #3 on: August 02, 2007, 08:55:38 AM »

I disagree that the S-meter reading should be lower because you're receiving less "noise" in a narrower bandwidth.

That might be true if the S-meter reading you're referring to is just background noise and not a signal.  But if you're referring to a drop in S-meter reading when you've tuned in a signal and switch to the narrower filter, noise has nothing to do with that.

The narrow filter could have more loss, though, and that would explain a lower S-meter reading.  Remember the IC-706 S-meter doesn't mean much, and "10 dB" indicated might really be 2 dB.  It's not accurate.  As long as you can still hear weak stations well, this shouldn't be a problem; however, I have rigs here where the S-meter reading doesn't change a bit if I switch from SSB 2.7 kHz bandwidth (or wider) down to 400 Hz, or even 250 Hz, on the same signal.  Same S-meter reading using any filter.  That is true with my old TR-7, my newer TS-850S and my still newer Jupiter.

As for the distortion: Distortion is in the ear of the listener, and what you think is distorted, I might not think so.  However, there are things that will add distortion: (1) If your noise blanker's turned on, try turning it off and see what happens.  (2) Use headphones and not a speaker and see what you think.  A lot of times the response is centered on about 600-700 Hz and using a narrow filter like 400 Hz, with response centered on 600 Hz, you'll only hear a signal having a pitch within a few hundred Hz of 600.  Lower frequencies can roll off to "nothing" audible, as will higher frequencies -- and much more rapidly than with a wider filter.  If the response center frequency is somewhere your speaker rattles or resonates, it can sound distorted.  Headphones can eliminate this. (3) Also, very strong signals can sound distorted; if the distortion you're describing occurs on very strong signals, try using the attenuator to make them weaker.

WB2WIK/6
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KE3WD
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Posts: 5694




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« Reply #4 on: August 02, 2007, 04:21:43 PM »

Practice matching your rig's sidetone pitch to the pitch of the signal you are listening to by fine tuning the Big Knob.  

Nailing the sidetone pitch also means nailing the frequency of the other station.  

Listening around 40 lately I notice that there seems to be a lot of hams who are not quite sure what the deal is with the sidetone, the offset, and how this really works.  It's simple stuff, read the manual, the ARRL publications, likely explanations abound on the web, too.  

The other night I listened to two guys chasing each other up the dial with each iteration, neither knowing what the heck was going on, just kept moving their freq as the other guy came back at a pitch they apparently did not approve of or something.  



.
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VK3GDM
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Posts: 28




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« Reply #5 on: August 05, 2007, 04:11:17 PM »

Thanks for the ideas fellas. I'll give these a try.
I think there is something a little suspicious going on tho.

Regards
David
VK3GDM
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N1UK
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Posts: 1383




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« Reply #6 on: August 07, 2007, 03:04:17 PM »

I have been using cwget and I find the audio frequency display invaluable for tuning in the other station spot on. If I set the cwget markers to 850Hz and center the other station on that then I am spot on. I find it amazing that most stations are at least 50 if  not up to 200 Hz off frequency. Is this because modern rigs are more difficult to zero beat.


Mark N1UK G3ZZM
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KE3WD
Member

Posts: 5694




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« Reply #7 on: August 07, 2007, 06:43:30 PM »

>>Is this because modern rigs are more difficult to zero beat. <<

I think it has more to do with "modern" hams not knowing how to match a pitch or what the zero beat/cw offset is even all about, or both, actually.  

Just listen to a group of people singing a simple song nowadays...


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